1. Field of the Invention
This invention relates to carburetors and more particularly to carburetors provided with inner and outer vents which are utilized in motor vehicle emission control systems.
2. Prior Art
In prior art carburetors, the inner vent communicates with the float chamber and with an air intake passageway upstream of the venturi-flume or choke valve so that evaporated fuel gas produced in the float chamber is discharged into an air intake passageway of the carburetor and burnt by the engine or the pressure in the float chamber is equalized with the pressure at a portion upstream of the air intake passageway. On the other hand, for the purposes of improving high temperature performance of the carburetor and suppressing the discharge of evaporated fuel gas, an outer vent is provided in the float chamber so that evaporated fuel gas produced in the float chamber is introduced into the charcoal cannister and the crank case via the outer vent when the engine is not being operated and thereafter is sucked out of the charcoal cannister and burnt by the engine when it is operating at some suitable time later.
However, as shown in FIG. 1, in the conventional carburetor 1, the inner vent 2 and the outer vent 3 are open to the float chamber 4. Accordingly, the inner vent 2 and the outer vent 3 communicate through the float chamber 4. When evaporated fuel gas produced in the float chamber 4 is sucked out of the charcoal cannister 5 which communicates with the float chamber 4 via outer vent 3, air which compensates for the sucked out gases flows from the air intake passageway 6 through the inner vent 2 into the float chamber 4. This air flow which passes through the inner vent 2, the float chamber 4 and outer vent 3 scavenges the float chamber 4 and facilitates the evaporation of the low boiling point components in the fuel gas. Accordingly, since the low boiling point components of the fuel gas are evaporated out of the fuel gas contained in the float chamber 4, there have been such drawbacks as starting failures, stalling, rough idling, etc. which occur immediately after the engine has been started.